Lab 1: Preparing Soap-Observing Intermolecular Forces
Laboratory Goals
In this lab, you will:
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Learn how soap is prepared
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Test some properties of soap
Safety Notes
1.
The sodium hydroxide solution used in this lab is extremely concentrated. Be sure to
avoid any contact with skin and especially eyes as it can cause serious burns. All spills
MUST be immediately reported to the LA and cleaned.
Introduction
The process of soap-making goes far back in history. Most people who have made soap
throughout the centuries have had no idea what is occurring; they simply made soap through trial
and error, lots of luck and governing superstitions.
The process (similar to what we will be doing in lab) involved combining some form of fat with
an alkali (basic) material. Most commonly the alkali was in the form of potash and pearlash,
which contain KOH. Potash and pearlash soaps were used by everyone from the reigning
monarchs to the peasant or cottager, who made their own soap from the waste fats and ashes they
saved.
The First Soap
It is recorded that the Babylonians were making soap around 2800 B.C. and that it was known to
the Phoenicians around 600 B.C. These early references to soap and soap-making were apropos
the use of soap in the cleaning of textile fibers such as wool and cotton in preparation for
weaving into cloth.
The Romans and Celtics
The first definite and tangible proofs of soap-making are found in the history of ancient Rome.
Pliny, the Roman historian, described soap being made from goat's tallow and causticized wood
ashes. He also wrote of common salt being added to make the soap hard. The ruins at Pompeii
revealed a soap factory complete with finished bars.
While the Romans are well known for their public baths, generally soap was not used for
personal hygiene. To clean the body the Greeks (and later the Romans) would rub the body with
olive oil and sand. A scraper, called a strigil, was then used to scrape off the sand and olive oil,
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also removing dirt, grease, and dead cells from the skin, which was left clean. Afterwards the
skin was rubbed down with salves prepared from herbs.
Throughout history, people have taken baths in a variety of bathing mediums, with herbs and
other ostensibly beneficial additives. It is well known that Cleopatra, who captivated the leaders
of the Roman world, attributed her beauty to her baths in mare's milk. During the early centuries
of the common era, soap was used by physicians in the treatment of disease. Galen, a second
century physician, recommended bathing with soap for the amelioration of some skin conditions.
Soap for personal cleansing became popular during the later centuries of the Roman era.
The Celtic peoples are also though by some historians to have discovered soap-making; their
soaps were used for bathing and washing. Perhaps due to increased contact with the Celtics by
the Romans, using soap for personal cleasing became popular.
There is an interesting legend surrounding the discovery of soap-making. This legend accords the
discovery of soap to the Romans, so it might have been fabricated to confront the Celtic claim to
soap-making. Probably both of these inventive peoples discovered soap-making independently.
The legend asserts that soap was first discovered by women washing clothes along the Tiber
River at the bottom of Sapo Hill. The women noticed their wash became cleaner with far less
effort at that particular location. What was happening? The ashes and the grease of animals from
the sacrificial fires of the temples situated on the top of Sapo Hill mixed with the rain; the
resulting soap--which ran down the slope in the streams of rain water--give the women a wash
day bonus. One can see at a glance that “saponification”, the chemical name for the soap-making
reaction, bears the name of that hill in Rome long ago, which caused one washer-women to
comment to another, "My wash is cleaner than yours".
The Chemistry of Soapmaking
As stated earlier, the chemistry behind soap-making was not understood for many years. It is
now known that saponification of soaps proceeds by the conversion of the triglycerides, which
are the components of fats and oils, to fatty acid salts and glycerol as show in Figure 1. The R
groups in the figure represent long carbon chains with the accompanying hydrogens. For each
specific triglyceride, these specific R groups can be determined. For example tristearin gives
the reaction shown in Figure 2.
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O
CH2OH
CH2OCR
CHOCR
O
∆
O
+
3 NaOH
3
NaOCR
+
CHOH
fatty acid salt
(soap)
O
CH2OH
CH2OCR
glycerol
a triglyceride
in a fat or oil
Figure 1: Reaction of a triglyceide with sodium hydroxide
O
CH2OH
CH2OC(CH2)16CH3
O
∆
O
+
CHOC(CH2)16CH3
3 NaOH
3
NaOC(CH2)16+
CH3
CHOH
Sodium stearate
O
CH2OH
CH2OC(CH2)16CH3
glycerol
tristearin
Figure 2: Reaction of tristearin with sodium hydroxide.
Typically, fats and oils have different more than one different R group in the same molecule so a
variety of sodium salts are produced. In order to separate out the salts from the rest of the
reaction products, a saturated NaCl solution is added. This forces the soap to coagulate without
dissolving in the water. It can then be collected by filtration and washed to remove the excess
base.
Table 1 lists the different carboxylic acids that originally reacted with glycerol to yield the
triglycerides found in the fats and oils. It shows the relative ratio of the acids for a variety of
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Table 1: Percent by weight of total fatty acids.
Mono
unsat.
Saturated
Oil or Fat
Poly
unsaturated
Unsat./
Linoleic Alpha
Sat. Capric Lauric Myristic Palmitic Stearic Oleic
Acid Linolenic
Acid
Acid Acid
Acid
Acid
Acid
ratio
(ω6)
Acid
(ω3)
C10:0 C12:0 C14:0
C16:0 C18:0 C18:1
C18:2
C18:3
Almond Oil
9.7
Beef Tallow
0.9
-
-
3
24
19
43
3
1
Butterfat (cow)
0.5
3
3
11
27
12
29
2
1
Butterfat (goat)
0.5
7
3
9
25
12
27
3
1
Butterfat (human)
1.0
2
5
8
25
8
35
9
1
Canola Oil
-
-
-
7
2
69
17
-
15.7
-
-
-
4
2
62
22
10
Cocoa Butter
0.6
-
-
-
25
38
32
3
-
Cod Liver Oil
2.9
-
-
8
17
-
22
5
-
Coconut Oil
0.1
6
47
18
9
3
6
2
-
Corn Oil (Maize Oil)
6.7
-
-
-
11
2
28
58
1
Cottonseed Oil
2.8
-
-
1
22
3
19
54
1
Flaxseed Oil
9.0
-
-
-
3
7
21
16
53
Grape seed Oil
7.3
-
-
-
8
4
15
73
-
Lard (Pork fat)
1.2
-
-
2
26
14
44
10
-
Olive Oil
4.6
-
-
-
13
3
71
10
1
Palm Oil
1.0
-
-
1
45
4
40
10
-
Palm Kernel Oil
0.2
4
48
16
8
3
15
2
-
Peanut Oil
Safflower Oil
4.0
-
-
-
11
2
48
32
-
10.1
-
-
-
7
2
13
78
-
Sesame Oil
6.6
-
-
-
9
4
41
45
-
Soybean Oil
5.7
-
-
-
11
4
24
54
7
Sunflower Oil
7.3
-
-
-
7
5
19
68
1
Walnut Oil
5.3
-
-
-
11
5
28
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Percentages may not add to 100% due to rounding and other constituents not listed.
Human depot fat, usually found in the abdomen of men and around the thighs and hips of
women, has a composition similar to lard.
fats and oils. Included under the name of each acid is the number of carbons in the molecule
along we the number of double bonds (eg. C12:0 represents 12 carbons with no double bonds.)
The structures for oleic acid and linoleic acid are shown below:
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O
CH3(CH2)7
(CH2)7COH
oleic acid
H
CH3(CH2)4
H
H2
C
O
(CH2)7COH
linoleic acid
H
H
H H
In this lab you will both make soap as well as test some of its properties. You can either use the
provide fats and oils, or you can bring in your own sample to make a different soap from
everyone else in lab (with which you can subsequently taunt them ☺.) Note that each of these
are shown in the cis configuration. The human body tends to deal with these in a much better
way than the trans-fatty acids.
Prelab
Write a purpose for the lab in you notebook. You might also wish to think about the
following questions. They will help you prepare for the lab, but you need not turn them in at the
beginning of class.
What reaction is described by the word saponification. What are the products from this
reaction and what are the required reactants?
Draw out the full structure for three of the carboxylic acids listed in table 1. Identify all
of the polar bonds, and find the polar and non-polar regions of the molecule.
What is the most dangerous part of the lab? Why is it critical to clean up all spills
immediately?
Procedure
First you will have to select the type of fat or oil that you wish to use to make soap.
Olive oil, vegetable oil, sesame oil and lard will be provided. You are welcome to use any other
type of oil or fat, but you will have to provide it (looking at other oils can give you drastically
different soaps that provide an interesting comparison.) Start with 20 mL of the selected oil (or
about 16 g of the lard) and put it in a clean 400 mL beaker. Add in 20 mL of ethanol followed
by 25 mL of 20 % sodium hydroxide (remember this is very concentrated. If any of it is spilled
it should be reported to immediately to the LA and then cleaned up. It is concentrated enough to
cause serious chemical burns and irreversible eye damage if it comes in contact with you.) Stir
the mixture with a glass rod.
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Turn on a hot plate and place the beaker atop it. Periodically stir the mixture during
heating. During the heating the mixture may foam up. Stirring will help prevent this, but if the
foam climbs up the beaker you will need to remove it from the heat momentarily until the
foaming subsides. The mixture should be heated until all of the ethanol is removed. When is
that? It will be when ethanol vapors are no longer being released from the heated mixture.
Being college students we will assume that you are familiar with the smell of ethanol (please
don’t test this by sticking your head over the beaker. This is just asking for trouble.) The loss of
the ethanol will likely coincide with the increase in the amount of foaming that is occurring in
your beaker.
Once the ethanol is gone remove the beaker from the heat and turn off the hot plate.
After allowing the mixture to cool most of the way to room temperature, add in 100 mL of
saturated sodium chloride and mix thoroughly. The soap should coagulate into a solid mass and
can now be filtered to remove the by-products. Our filtering will be done using a wire screen
(instead of filter paper). To do this put the wire screen on top of a large beaker and pour the soap
containing liquid onto the gauze (some soap may get through, but the majority will collect on the
gauze.) The soap should then be washed at least twice with 10 mL of ice water (ice can be
obtained from the biology lab) to remove the excess NaOH. It may be more effective to put the
soap back into the original beaker and add the ice water and filter again. Once the rinsed soap
has drip dried, move it to a paper towel to finish drying.
Testing the Soap
Take a pea sized piece of the newly formed soap and put it in a 125 mL Erlenmeyer flask
and add about 50 mL of distilled water. Repeat this in a second and third flask using either a
commercial soap or soap made by another lab group starting from a different fat or oil. Stopper
the flasks and shake them vigorously for 20-30 seconds and observe the results paying attention
to the solubility of the soaps and the foaming action. The ability to create a foam indicates the
presence of the soap. The amount of foaming and the length of time until deflation both relate to
the surface tension of the solution.
Once you have been able to compare the rate of deflation of the foam, take a clean glass
rod and dip it into a solution and then touch a piece of pH paper. Test the other solutions as well
as the pH of distilled water. This will give a feeling of how well you were able to remove the
excess NaOH.
Next add two drops of mineral oil to the solutions. To another flask add 50 mL of
distilled water and 2 drops of mineral oil. Cover and shake the flasks for about 10 seconds and
compare how well the soaps were able to emulsify the oil (prevent it from immediately
separating out.)
Last, clean out the flaks and rinse with distilled water. Put a pea sized piece of your soap
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and another group’s soap in a flask and add in 10 drops of a 5% solution of calcium chloride.
Calcium is one of the common components in hard water. Shake the flask for about 10 seconds
and observe the changes. This reaction leads to “ring around the tub” and a dull grey on clothes.
Questions
1. What is an emulsion? Which mixture was the better at maintaining an emulsion, oil and
water or soap, oil and water? Explain why on a molecular level.
2. How can soap remove oil or dirt from clothes?
3. What was the role of the ethyl alcohol that was originally added to the reaction mixture?
4. Explain chemically what happens when soap is added to hard water and the problems that
can occur when washing.
Writing
This lab report will be a formal lab report written with a partner. Try to answer the above
questions as a part of your discussion or introduction section, rather than in independent
“question” section. It will both make the lab report flow better as well as really give you
something to “discuss” in your discussion section.
Chemicals:
20% NaOH, ethanol, saturated NaCl, 5% calcium chloride, vegetable oils, mineral oil, soap
Chemical Disposal
All liquid chemicals may be disposed of down the drain. NaOH should be washed down the
drain with lots of water. Any solid waste (excess lard or soap that you don’t want to keep)
should be discarded in the trash.
References
1. “Fats, Oils, Fatty Acids, Triglycerides - Chemical Structure” a webpage by
ScientificPsychic.com accessed Nov 2, 2004 at
http://www.scientificpsychic.com/fitness/fattyacids.html
2. “Colonial Soap-making” a webpage by Alcasoft
http://www.alcasoft.com/soapfact/history.html accessed Nov 2, 2004.
3. Paul Kelter, Jim Carr and Andrew Scott, Laboratory Manual to accompany Chemistry a
World of Choice, McGraw-Hill, Boston, 1999.
Special thanks to Anna Wasson for grammatical editing.
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